After LUX: The LZ Program. David Malling, Simon Fiorucci Brown University APS DPF Conference August 10, 2011

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1 After LUX: The LZ Program David Malling, Simon Fiorucci Brown University APS DPF Conference August 10, 2011

2 The LZ Program LZ LUX-ZEPLIN LUX (14 U.S. institutions) + new collaborators from ZEPLIN, other U.S. institutions Two phases LZ-S (1.5T or 3T) Construction late 2012; running LZ-D (20T) Construction 2014; running 2018 onward 2

3 LZ at a Glance 3

4 LUX Innovations for LZ Davis Cavern infrastructure, water shield: ready for up to 3 ton instrument Heat exchanger, high flow rate Xe purification system Remote feedthroughs and cryogenics Low-background titanium cryostat Scalable internals construction Scalable trigger and DAQ (DDC-8) 83m Kr, 3 H calibration sources Automated Control and Emergency Recovery systems Safety review process 4

5 LZ Innovations 3" PMTs at <1 mbq 238 U+ 232 Th Liquid scintillator shield/veto Internal active plastic veto Internal imaging system 5

6 Scintillator Shield/Veto Scintillator housed as close as possible to LXe Ti cryostat especially helpful, want ~1 cm thickness Cold (175 K) placement, immediately outside LXe Highest efficiency Likely choice: iso-hexane + flour. Expect factor 2-3 less light than pseudocumene; less flammable Warm: better cryo safety, reduction in efficiency Program of low temperature scintillator study, combined with MC studies Goal: <x1/10 reduction of gamma, neutron rates in LXe Final decision on scintillator veto option based on performance, safety 6

7 PMTs LZ PMTs will have 3 diameter 2x photocathode surface area compared to LUX R8778s Initial 3 testing: comparable performance to R8778s in LXe (QE, gain, single phe resolution) QE to be improved with super bialkali photocathodes R11410 MOD: Measured < U / < Th mbq/pmt <x1/15 EM, <x1/18 neutron emission from R8778 R8778 R11410 MOD R11065 single phe calibration -100 C, V Temp = 173 K Gain = 7.40(0.03)e6 σ/µ = Potentially used in LUX (2x 31-PMT arrays) 7

8 Architecture developed for LUX 2 years operational experience on full-size prototype (LUX 0.1). ~70 thermometers, 5 PID control points. Liquid nitrogen (LN) thermosyphon backbone Extremely high capacity, remotely deployed, multiple cold heads, tunable to low power for fine control Intrinsically safe: passive, insensitive to power loss Probable LN generation on site to avoid LN transport Conventional system for precooling scintillator Cryogenics 8

9 Internals Large area grid prototyping Scale will increase from 0.5 m to 2m and maintain acceptable deflection Grids LZS LUX Low mass field ring development Minimize mass for veto Investigation of active plastic to enhance veto capability LXe compatibility Maximize light collection Development of internal imaging system for enhanced monitoring Internal fiberscope to view liquid surface and components PMTs for Internal Veto Scin%llator Bars 9

10 Calibrations External gamma sources unable to penetrate tonne-scale detectors -- require internal sources Two methods developed for LUX to be used in LZ: arxiv:0905: m Kr: 1.8 hr half-life Tritiated methane (CH 3 T) First test of removal from LXe: >90% Energy calibration: 83m Kr 55 Fe x-rays Xe purifier tritium spectrum after purification Electron recoil discrimination: 3 H Proportional tube for tritium counting backgrounds 99.99% removal from gas (arxiv:1002:2791) 10

11 External Backgrounds LZ-D uses 12m x 12m water shield 10 9 LZ-D External n bkgd No scint. LZ20 Background Events from External Neutrons kev r Water shield alone reduces fast neutron BG <1 WIMP-like evt / 1000 days cts/kevr/kg/day / r / kg / All NR Events SS SS, elastic SS, elastic, fiducial Addition of scintillator veto: <x1/100 further suppression E dep [kev r ] Edep (kevr) LZ-D External n bkgd Scint. only LZ20 Background Events from External Neutrons kev r Scintillator Veto (100 kev ee threshold) Further factor of x1/100 reduction from standard analysis cuts Comparable reductions in neutrons produced in water shield itself cts/kevr/kg/day / r / kg / All Events Scintillator veto ( % reduction) E dep [kev r ] Edep (kevr) 11

12 Internal Backgrounds Low-energy, single-scatter requirement: backgrounds dramatically suppressed in detector center NR Activity vs. Fiducial Mass With increase in total detector mass, drbg/dmfid steepens substantially 13.5T fiducial set by requirement of <1 WIMP-like ER event / 1000 days from R8778-style tubes LUX LZ-S LZ-D <1 mbq 238 U/ 232 Th tubes allow ~17T fiducial External scintillator provides additional rejection for escaping gammas and neutrons LZ 20T 3 PMT ER Backgrounds 5-25 kevee, singlescatter, scint. veto 12

13 Cosmogenic Backgrounds LZ-D Xe mass x67 above LUX -- must search for cosmogenic products previously overlooked Naked β β Xe activation by muon capture, neutron capture, fast neutron activation, etc.: >200 isotopes produced 137 Xe 137 Cs Only worry about naked or seminaked beta emitters γ β Semi-naked β ~10-7 /kev/kg/day event rate, primarily from fast neutron activation ( 137 Xe) 137 Xe 137 Cs 13

14 Neutrino Backgrounds Dark matter signal search fundamentally limited by neutrinos Electron recoil signal limited by p-p solar neutrinos LZ-D: 5 evts (1-10 kev ee) / 1000 days before ER rejection Neutron recoil signal limited by coherent neutrino scattering 8 B DSNB Atmospheric LZ-D: ~1 evt (5-25 kev r) / 1000 days cts/kevee/tonne/1000 days cts/kevr/tonne/1000 r /tonne/1000 days days kevee 10 8 B solar atmospheric PMT (,n) 1/1 mbq U/Th 8 B solar E res. conv. 100 GeV WIMP (1e 48 cm 2 ) 100 GeV WIMP (1e 47 cm 2 ) DSNB 10 kevr Nuclear Recoil Energy (kev r ) 14

15 SI WIMP Sensitivity Projections based on background studies, previously attained e- attenuation lengths and discrimination factors Fiducial volumes selected to match <1 NR event in experiment lifetime LUX (black): 100 kg x 300 days LZ-S (cyan): 1200 kg x 600 days LZ-D (purple): kg x 1000 days Cross section [cm 2 ] (normalised to nucleon) LUX (1 BG evt) LZ-S (1 BG evt) 10 BG evts 1 BG evt LZ-D Gaitskell,Mandic,Filippini WIMP Mass [GeV/c 2 ] 15

16 Summary LZ tonne-scale Xe detectors will use technology tested in LUX Cryogenics, purification, low-background construction materials, internal calibration sources, etc. Combination of external scintillator and water shield reduces external backgrounds to levels subdominant to internal backgrounds for both LZ-S and LZ-D LZ-D will push LXe dark matter detection to its final limit from neutrino signals 16